In integrated steel plants, large quantities of waste materials are produced from air and water treatment facilities within the plant. Wastes generated during operation of these facilities must be collected and treated prior to discharge, in order to meet applicable Environmental Protection Agency ("EPA") discharge criteria. There exist numerous methods for treating and disposing of accumulated wastes.
For example, in my prior U.S. Pat. No. 5,435,835, I disclose a method for chemically stabilizing lead and zinc contaminated steel-making waste. Zinc and lead containing wastes may be classified as hazardous wastes, and must pass the toxicity characteristic leaching protocol (TCLP) test mandated by the EPA. The TCLP test requires that lead (the constituent of concern) levels be below a prescribed 5 ppm level. In order to meet this test, it was found that a 4-8% by weight lime addition would precipitate sufficient lead and also zinc to meet the applicable level. Thus, my prior method is extremely beneficial in reducing zinc and lead quantities in a wastewater stream, and that the resulting slurry passes the TCLP test and is considered nonhazardous.
However, the production and disposal of hazardous waste is not the only or even the primary environmental concern at a steel plant. In addition to the potential hazardous waste produced at steel plants, thousands of tons of nonhazardous waste are also produced. Nonhazardous waste is defined as waste not classified as hazardous under the TCLP test. The nonhazardous waste must pass the paint filter test mandated by the EPA in order to be disposed of in a landfill. The paint filter test requires that a sample of waste be absent of any free water before it becomes landfillable. Thus, solids and liquids must be effectively separated in order for the solids to be disposed of in a landfill. Collected slurries typically contain extremely fine particles, with a low solids concentration, and may also contain oil. Because of the low solids concentration, it becomes difficult to both filter and agglomerate the solids. In addition, the oils contained in the slurry may cause filter cloth blinding, which clogs the filter and prevents effective liquid/solids separation. Thus, dewatering a slurry takes a long time, and may be costly. For instance, in order to filter the slurry at a high rate, larger filtering equipment may be necessary, which then results in significant additional costs.
Even with the addition of large filtering equipment, the nonhazardous waste is accumulated at such a high rate that it is difficult to dispose of the waste as quickly as it is accumulated. There has yet been a method developed which can filter the nonhazardous waste at a rate equal to the rate at which it is accumulated, without incurring outrageous operating cost.
For instance, the slurry may be heated to decrease the slurry viscosity, for enabling more effective liquid/solids separation. However, heating thousands of tons of nonhazardous waste is both expensive and extremely impractical.
Finally, the agglomerated solid mass that has been filtered and dewatered, and is ready for landfill disposal, may be thixotropic and become liquefied upon transportation to a landfill. As such, the material may not be disposed of in a solid waste landfill, because it will not pass the paint filter test upon arrival to the landfill site.
Therefore, there exists a need in the art for a method of improving the filtration rate of nonhazardous iron and steel making wastes, in which the filtered solids do not become liquefied during transport. It is a purpose of this invention to fulfill this and other needs in the art which will become more apparent to the skilled artisan once given the following disclosure.